Carbon Monoxide (CO) Detection and Fire Alarm Systems
Carbon monoxide comes up in two completely different fire-safety conversations, and mixing them up is a common and important mistake. On one hand, CO can be used as a signature to help detect fire; on the other, CO alarms protect people from poisonous gas leaking from faulty appliances. This guide explains the difference, where CO fire detection fits, and why one never replaces the other.
Getting this distinction right matters for both safety and compliance, so it is worth being clear from the outset.
Who this is for
This is for competent fire alarm engineers who encounter CO or multi-sensor detectors, or are asked about CO alarms. The experience level assumed is competent engineer. Use it for the concepts; specific detector selection and CO-alarm requirements come from the fire risk assessment, BS 5839-1, the relevant toxic-gas standard, and manufacturer data.
Two different CO devices
There are two distinct devices that both sense carbon monoxide. A CO fire detector uses CO as one indicator of a fire, almost always within a multi-sensor detector on the fire alarm system, improving fire detection performance and false-alarm rejection. A CO alarm is a separate life-safety product, made to the toxic-gas alarm standard, that warns occupants of dangerous CO from faulty appliances, flues or combustion. They look superficially similar but serve entirely different purposes and are governed by different standards. Confusing them — for example assuming a fire-system CO detector protects against appliance CO — is a genuine safety error.
Why CO helps fire detection
Many fires, particularly slow smouldering ones, release carbon monoxide early in their development. A multi-sensor detector that combines CO sensing with optical and, often, heat sensing can respond well to such fires while rejecting more of the transient nuisances that would trip a single optical head. That improved balance of sensitivity and false-alarm immunity is why CO is included in some multi-sensor detectors. Whether to specify CO-based multi-sensor detection is a design decision for the environment, applied within a fire alarm system designed to BS 5839-1.
CO fire detectors do not replace CO alarms
This is the crucial point. A CO fire detector on the fire alarm system exists to detect fire — not to protect occupants from toxic CO produced by a faulty boiler, heater or blocked flue. Protection against appliance-related CO poisoning is the job of dedicated CO alarms to the relevant toxic-gas standard, driven by gas-safety and building requirements. A building may need both, for different reasons. Never present a fire-system CO detector as CO poisoning protection; the fire risk assessment and the appliance/gas-safety requirements determine what is actually required.
Maintenance and service life
CO sensing elements do not last forever — they lose sensitivity with age, so manufacturers set replacement intervals for CO and multi-sensor detectors, often shorter than for a plain optical head. Maintenance follows the detector manufacturer's method within the normal fire alarm servicing regime. When servicing, confirm the exact device type, check its recommended replacement age, and test it by the manufacturer's procedure. From field experience, CO-bearing multi-sensors left in service past their replacement age are a recurring find.
Common points to check
Recurring issues include CO or multi-sensor detectors past their service life, and confusion on site about whether a device is a fire-system CO detector or a standalone CO alarm. Confirming the device type and its purpose avoids both safety and compliance mistakes.
When not to rely on this alone
When not to use this article: do not use it to decide CO-alarm provision for appliance safety, or to select fire detectors for a specific building. Those come from the fire risk assessment, gas-safety and building requirements, BS 5839-1, the relevant toxic-gas standard, and manufacturer data, applied by competent professionals.
Relevant standards
CO-based fire detection sits within BS 5839-1, a code of practice for fire detection systems, while CO alarms for appliance safety follow the relevant toxic-gas alarm standard. The legal duty for fire precautions in most non-domestic premises sits under the Regulatory Reform (Fire Safety) Order 2005, with Building Regulations statutory guidance in Approved Document B applying to building work. Separate the legal duty from the recommended methods when advising a client, and always work to current editions.
Professional disclaimer
This is an educational resource for competent engineers and does not replace the current British Standards, the fire risk assessment, gas-safety requirements, manufacturer data, or competent judgement. Verify CO detection and alarm decisions against current documentation.
Related documentation
Use this with the current BS 5839-1, the detector manufacturer's data, and the fire risk assessment. Where appliance CO safety is involved, refer to the relevant gas-safety and toxic-gas alarm requirements. Record detector types and replacement ages in the service documentation.